Efficient Silicon Solar Cells Fabricated with a Low Cost Spray Technique 83 3. Brief description of the film properties 3.1 Tin-doped indium oxide (ITO) films The X-ray diffraction (XRD) measurements shown in Figure 1 indicate that all deposited ITO films, with thickness 160-200 nm and fabricated from the chemical solutions with different Sn/In ratio, present a cubic bixebyte structure in a polycrystalline configuration with a (400) preferential grain orientation. 10 20 30 40 50 60 70 0 2000 4000 6000 8000 (622) (611) (440) (411) (400) (222) T=480 °C [Sn]/[In]=0 % [Sn]/[In]=5 % [Sn]/[In]=11 % Counts (a. u.) 2 θ (grad) Fig. 1. XRD spectra of the ITO films fabricated from precursors with different Sn/In ratio The average size of the grains, 30-50 nm, was determined using the classical Debye-Scherrer formula from the half-wave of the (400) reflections of the XRD patterns A surface roughness about 30 nm was determined from images of the films surfaces obtained with the atomic force microscope (Figure 2). Fig. 2. AFM images of the In 2 O 3 film (left) and the ITO film with 5% Sn/In (right) Figures 3 and 4 show the dependence of electric parameters of the spray deposited ITO film on the ratio Sn/In. The sheet resistance R s shown in Figure 3 presents a minimum of 12 Ω/□ the films prepared from the solution with a 5% Sn/In ratio. Solar Energy 84 Fig. 3. The sheet resistance as a function of the Sn/In ratio in the precursor used for the film deposition. The thicknesses of the films are also shown The minimal value of resistivity obtained for the films deposited for the solution with 5% Sn/In ratio is 2×10 -4 Ω-cm. The variation of mobility and carrier concentration as a function of the Sn/In ratio are shown in Figure 4. Fig. 4. Dependence of mobility (μ) and carrier concentration (n) on the Sn/In ratio Figure 5 shows the optical transmission spectra for the ITO films spray-deposited on a sapphire substrate as a function of the wavelength for solutions with different Sn/In contents. The use sapphire substrates allow for determining the optical energy gap of the ITO films by extrapolating the linear part of α 2 (hν) curves to α 2 =0, where α is the absorption coefficient. Efficient Silicon Solar Cells Fabricated with a Low Cost Spray Technique 85 400 600 800 1000 1200 0 20 40 60 80 100 c b a T=480°C a- Sn/In=0 b- Sn/In=5% c- Sn/In=11% Percentage tracmission Wavelength [nm] Fig. 5. Optical transmission spectra for the ITO films spray-deposited for different precursors as a function of the wavelength The optical gap increases with the carrier concentration, corresponding to the well known Burstein-Moss shift. For the Ito films fabricated using the solution with a 5% Sn/In ratio this shift is 0.48 eV, and the optical gap is 4.2 ± 0.1 eV. Such high value for the optical gap offers transparency in the far ultraviolet range, which is important for the application of these films in solar cells. Because of the opposite dependence of the conductivity (σ) and transmission (T) on the thickness (t) of the ITO, both parameters need to be optimized. A comparison of the performance for different films is possible using the φ TC =T 10 /R s =σt exp(-10αt) figure of merit (Haacke, 1976). Table 1 compares the values of φ TC for the spray deposited ITO films reported in this work with some results obtained by other authors using different deposition techniques. Process R s , Ω/□ T (%) φ TC, (Ω -1 ) ×10 -3 Author spray 26.0 90 13.4 Gouskov, 1983. spray 9.34 85 21.0 Vasu et al., 1990 spray 10.0 90 34.9 Manifacier, 1981 spray 4.4 85 44.7 Saxena, 1984 sputtering 12.5 95 47.9 Theuwissen, 1984 evaporation 25.0 98 32.6 Nath, 1980 spray 12.0 93.7 43.5 Present work Table 1. Comparison of the values of φ TC for ITO films 3.2 Fluorine-doped tin oxide (FTO) films The X-ray diffraction (XRD) measurements indicate that all the spray-deposited FTO films present a tetragonal rutile structure in a polycrystalline configuration with a (200) Solar Energy 86 preferential grain orientation. The XRD spectra of the FTO films fabricated using precursors with different F/Sn ratios are shown in Figure 6. 0 204060 0 5 10 15 20 25 30 F/Sn =0 F/Sn=0.35 F/Sn=0.50 F/Sn=0.65 F/Sn=0.85 F/Sn =1 (301)(310) (211) (200) (110) Counts (x10 3 ), a.u. Angle of diffraction 2 θ (degree) Fig. 6. The XRD spectra for the FTO films fabricated using precursors with different F/Sn ratio The surface morphology of the films fabricated using precursors with different F/Sn ratio, and obtained with a scanning electron microscopy (SEM), is shown in Figure 7. Fig. 7. The surface morphology obtained with a SEM for the films fabricated using precursors with different F/Sn ratios The dependence of the average value of the grain size on the F/Sn ratio shows a maximum (∼ 40 nm) for the films prepared using a precursor with F/Sn=0.5. The roughness variation Efficient Silicon Solar Cells Fabricated with a Low Cost Spray Technique 87 obtained with atomic force microscope for the FTO film fabricated using solutions with different F/Sn ratios presents a minimum of 8-9 nm at the F/Sn=0.5 ratio. Figure 8 shows that the electrical characteristics also present some peculiarities for the films prepared using a precursor with this F/Sn ratio. Fig. 8. Variation of the sheet resistance (above graph), resistivity (ρ), mobility (μ) and carrier concentration (n) (below graph) for the FTO films fabricated using precursors with different F/Sn ratios. The thicknesses of the films are also shown Solar Energy 88 200 400 600 800 1000 0 20 40 60 80 100 b c a Transmittance [%] Wavelength [nm] a- F/Sn-0 b- F/Sn-0.50 c- F/Sn-0.85 0.00.20.40.60.81.0 4.3 4.4 4.5 4.6 4.7 F/Sn ratio in solution E g opt [eV] Fig. 9. Optical transmission (above graph) and dependence of the optical gap (below graph) for the FTO films fabricated using solutions with different F/Sn contents and spray- deposited on a glass substrate as a function of the wavelength The optical energy gap (Fig. 9) was determined from the analysis of the absorption spectra for the films deposited on the sapphire substrate. The Burstein-Moss shift presents a Efficient Silicon Solar Cells Fabricated with a Low Cost Spray Technique 89 maximum value of 0.6 eV for the films fabricated using the precursor with F/Sn =0.5, which also corresponds to the highest electron concentration (1.8×10 21 cm -3 ). Figure 10 shows the Φ=T 10 /R s figure of merit for the FTO films reported in this work. 0.0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 F/Sn ratio in solution φ TC [10 -3 , Ω -1 ] Fig. 10. Variation of the figure of merit Φ=T 10 /R s versus the F/Sn ratio used in the solution for the FTO films reported in this work The value we obtained for this figure of merit was Φ =75×10 -3 Ω -1 for the films prepared using a precursor with F/Sn =0.5; this is more than twice the value (Φ =35×10 -3 Ω -1 ) reported in the literature (Moholkar et al., 2007) for spray deposited FTO films. 4. Solar cells based on ITO/n-Si heterojunctions 4.1 Physical model of the solar cells When the ITO (or FTO) film is deposited on the silicon surface, a metal-semiconductor contact-like is formed due to the metallic electric properties of the degenerated metal oxide. Ideally, the barrier height (ϕ b ) formed between the metal and the n-type semiconductor is determined by the difference between the metal (or in our case the metal oxide) work function (ϕ M ) and the electron affinity (χ s ) in the semiconductor. Actually, the surface states present in the interface pin the Fermi level, which makes the barrier height less sensitive to the metal work function (Sze, 2007). The surface has to experiment a reconstruction due to the discontinuity of the lattice atoms on the surface. Each surface atom present a dangling bond and shares a dimer bond with its neighbor atom, thus giving place to surface states inside the Si band gap (Trmop, 1985). Recently, it has been shown that the barrier height in a metal-silicon junction can take an almost ideal value if the n-Si surface is passivated with sulfur (Song, 2008). Also the open- circuit voltage of an Al/ultrathin SiO 2 /n-Si solar cell (Fujiwara, 2003) was improved when the silicon surface was passivated by a cyanide treatment. Solar Energy 90 In this chapter we will discuss the properties of the ITO/n-Si solar cells presenting extremely high values of the potential barrier at the silicon interface obtained by passivating the surface with a hydrogen-peroxide solution. If the ITO film is deposited on cleared n-type silicon, the barrier height not exceeds 0.76 eV. For this value of the barrier height, the ITO/nSi heterojunctions fabricated on silicon substrates with a resistivity of a few Ω-cm, operate as majority carrier devices, whose characteristics are well described by the Schottky theory. Usually, such type of devices present a high value for the dark current originated by the thermo-ionic mechanism, and the open circuit voltage for these structures designed as solar cells shows a sufficiently low value. The introduction of a very thin (∼ 2 nm) intermediate SiO x layer (Feng, 1979) decreases the dark current and increases the open-circuit voltage. However, the use of this approach to improve the characteristics of the surface-barrier solar cells requires a simultaneous and careful control of the intermediate oxide thickness. Furthermore, the thermal grown intermediate SiO x layer always presents a positive fixed charge located at the SiO x /Si interface, which decreases the barrier height in the case of n-type silicon. Using known data for the work function of ITO films deposited by spray pyrolysis, whose average value is reported as 5.0 eV (Nakasa et al., 2005, Fukano, 2005), and the electron affinity of silicon as 4.05 eV, the ideal barrier height between ITO and n-type silicon is 0.95 eV according to the Mott-Schottky theory. After a treatment of the n-type silicon surface in the hydrogen-peroxide (H 2 O 2 ) solution with a controlled temperature (60 0 C) during 10 minutes, a barrier height of 0.9 eV was obtained with capacitance-voltage measurements. This value exceeds by 0.14 eV the barrier height obtained after the deposition of the ITO film on the silicon surface cleaned in HF without the treatment in an H 2 O 2 solution. It is worth discussing the possible reason for this increment of the barrier height after the treatment of the silicon surface, as well as the operation of the ITO/n-Si junctions with an extremely high barrier height. Obviously, a junction with such barrier height fabricated on the silicon substrates with moderate resistivity could behave as p-n junctions, in which a surface p-layer is induced by the high surface band bending. Such situation was obtained (Shewchun, 1980) in solar cells ITO/ultrathin SiO x /p-Si structures. However, in this case the inversion of the conductivity type of the p-Si at the surface was caused by other factors, such as the low work function of the sputtered ITO film and the presence of positive charge at the SiO x /p-Si interface. What is the physical reason for the increment of the barrier height in the ITO/n-Si heterojunctions after the treatment of the silicon substrate in heated 30% H 2 O 2 solutions? It has been shown (Verhaverbeke, 1997) that the treatment of the silicon in H 2 O 2 leads to the growth of oxide on the silicon surface. The analysis shows that the main oxidant responsible for this oxide growth is the peroxide anion, HO 2 ¯ . It was also found that the oxide thickness is limited to a value around 0.8-1.0 nm due to the presence of localized negative charge (HO 2 ¯ ) at the silicon surface. From this point of view the HO 2 ¯ at the silicon surface can play a double role. First, these ions can form a chemical composition with the silicon atoms having dangling bounds in the surface. This can be thought as a passivation of the silicon surface, which leads to an increment of the potential barrier during the formation of the ITO/Sl heterostructure. On the other hand, the negative charge of these ions can produce a band-bending (φ s ) at the silicon surface due to an outflow of electrons under the influence of the electrostatic force. Under such conditions, the electron affinity (χ s ) of the silicon at the surface will be lower than that at the bulk by Δχ=χ s -φ s . The presence of a depletion layer at Efficient Silicon Solar Cells Fabricated with a Low Cost Spray Technique 91 the silicon surface plays an important role for the formation of the potential barrier during the deposition of the ITO film. The barrier will prevent an electron flow from the silicon to the ITO film. The surface barrier between the ITO and the silicon will be formed by the flow of valence electrons from the silicon valence band into the ITO film, creating a hole excess at the silicon surface. Taking into account the initial band-bending at the silicon surface, the formation of an inversion layer is possible. As it was already mentioned, the experimentally determined barrier height at the ITO/Si interface is 0.9 eV. Schematically, the energy diagram of the ITO/n-Si heterojunction is shown in Figure 11. Fig. 11. Energy diagram (in kT units) of the heavy doped ITO/n-Si heterojunction For sake of simplicity, we do not show the very thin (around 1 nm) intermediate SiO x layer present between the ITO film and the silicon, because at this thickness it does not present any effect on the electro-physical characteristics of the heterojunction. Since the heavily doped ITO film is a degenerated semiconductor, in which the Fermi level lies above the minimum of the conduction band, we can consider this ITO film as a “transparent metal.” The inversion layer at the silicon interface appears when the barrier height φ b is higher than one-half of the Si energy gap. If such inversion p-n junction were connected in a circuit, which source of holes would be present in order to form an inversion p-layer that complicates the current flow across the forward-biased structure working as a solar cell? To answer this question we calculated the number of empty energy states in the conduction band of a heavy doped ITO, which are available to accept the electrons transferred from the top of the silicon valence band located at a distance Δ below the Fermi level (Malik et al., 2006). The probability that an energy state E below the Fermi level E FM in the degenerated ITO is empty was calculated using the Fermi-Dirac distribution. Using a barrier height φ n =0.9 eV, Δ=0.3 eV, and three different values for (E FM -E CM ), which is the distance between the Fermi level an the conducting band of the ITO. This characterizes the degree of degeneration of the ITO film. The calculated number of empty states available to accept the E FM /kT E CM /kT=0 Δ E/kT E CS /kT E FS /kT ξ (ITO) φ b /kT eφ p /kT x = 0 x E opt g /kT E VS /kT Solar Energy 92 electrons from the silicon valence band forming the additional amount of the holes is shown in Figure 12 as triangles. For comparison the number of empty states in the case of a gold/silicon contact with the same barrier height is also shown. For such calculations, the difference between the effective mass of electrons in the ITO and that in gold has been taken into account. Fig. 12. Calculated number of empty states available to accept the electrons from the silicon valence band (Malik et al., 2006) From the discussion presented above, and the amount of the calculated number of empty states in the ITO, leads to the important conclusion that a heavy doped ITO layer serves as an efficient source of holes necessary to form the inversion p-layer in the ITO/n-Si structures. 4.2 Evidence of the inversion in the type conductivity in the ITO/n-Si heterostructures Based on the barrier height (0.9 eV) obtained from the measured C-V characteristics for the ITO/n-Si heterostructures on 10 Ω-cm monocrystalline silicon, one can discuss about the physical nature of such heterojunctions. Because the barrier height exceeds one half of the silicon band gap, the formation of an inversion p-layer at the silicon surface is obvious from the band diagram. To avoid any speculations on this issue and in order to present a clear evidence for the existence of a minority (hole) carrier transport in these heterojunctions, a bipolar transistor structure was fabricated on a 10 Ω-cm monocrystalline silicon substrate, in which the emitter and the collector areas, on opposite sides of the silicon substrate, were fabricated based on the ITO/n-Si junctions. The ITO film was deposited using the spray deposition technique described in section 2.1 followed by a photolithographic formation of the emitter and the collector areas. The treatment in the H 2 O 2 solution described above was applied to the silicon substrate. An ohmic n + -contact (the base) was formed using local diffusion of phosphorous in the silicon substrate. The dependence of the collector current versus the collector-base voltage, using the emitter current as a parameter, are shown in [...]... Non-Cryst Sol., Vol.3 54, 247 2- 247 7, ISSN 0022-3093 Manifacier, J & Szepessy, L (1977) Efficient sprayed In2O3:Sn n-type silicon heterojunction solar cell Appl Phys Lett., Vol.31, N.7, 45 9 -46 2, ISSN 0003-6951 Manifacier, J.; Fillard, J & Bind J (1981) Deposition of In2O3-SnO2 layers on glass substrates using a spraying method Thin Solid Films, Vol 77, N.1-3, 67-80, ISSN 0 040 -6090 1 04 Solar Energy Moholkar,... silicon Appl Phys Lett., Vol.29, N.8, 49 4 -49 6, ISSN 0003-6951 Feng, T.; Ghosh, A & Fishman, G (1979) Efficient electron-beam-deposited ITO/n-Si solar cells, J Appl Phys., Vol.50, N.7, 49 72 -49 74, ISSN 0022-3727 Fukano, T.; Motohiro, T & Ida, T (2005) Ionization potential of transparent conductive indium oxide films covered with a single layer of fluorine-doped tin oxide nanoparticles grown by spray pyrolysis... Microelectronics (ICM 20 04) , pp 47 1 -47 4, ISBN 0-7803-8656-6, Tunis, December 06-08, 20 04, IEEE, Tunisia Malik, O.; Grimalsky, V & De la Hidalga-W, J (2006) Spray deposited heavy doped indium oxide films as an efficient hole supplier in silicon light-emitting diodes J NonCryst Sol., Vol.352, 146 1- 146 5, ISSN 0022-3093 Malik, O.; De la Hidalga-W, J.; Zúñiga-I, C & Ruiz-T, G (2008) Efficient ITO-Si solar cells and... McCandless et al., 2003; Ferekides et al., 20 04) In the long-wavelength region, the spectra are restricted to the value λg corresponding to the band gap of CdTe which is equal to 1 .46 eV at 300 K (λg = hc/Eg = 845 nm) In the short- 107 Efficiency of Thin-Film CdS/CdTe Solar Cells 0 .4 1.0 246 K 271 K 0.8 TITO 293 K TITO, TCdS η (λ) 0.3 315 K 0.2 0.6 TCdS 0 .4 336 K 0.1 0.2 356 K 0 0 300 500 λ (nm) 700... However, if the depletion 111 Efficiency of Thin-Film CdS/CdTe Solar Cells (a) 1.0 15 τn = 10–10 s 10 0.8 Na − Nd = 10 14 cm–3 1016 0.8 10 14 0.6 η(λ) (b) 10–6 10–8 10–9 10–10 0.6 1013 η(λ) 1.0 0 .4 0 .4 τn = 10–11 s Na − Nd = 1017 cm–3 0.2 0 0.2 300 500 λ (nm) 700 0 900 300 500 λ (nm) 700 900 (c) 1.0 0.8 10 16 cm –3 ηint(λ) 0.6 10 15 cm –3 10 14 cm –3 0 .4 0.2 0 τ n = 10 –10 s 300 500 λ (nm) 700 900 Fig 3 (a)... coating on GaSb or Ga1-xAlxSb for IR photodetection Thin Solid Films, Vol.99, N .4, 365-369, ISSN 0 040 -6090 Haacke, J (1976) New figure of merit for transparent conductors J Appl Phys., Vol .47 , 40 8 640 89, ISSN 0022-3727 Hamberg, J & Granqvist, C (1986) Evaporated Sn-doped In2O3 films: basic optical properties and applications to energy- efficient windows J Appl Phys., V.60, n.11, R13, ISSN 0022-3727 Hartnagel,... theory of the p-n junction based solar cells for modelling the ITO/n-Si solar cells with a barrier height of 0.9 eV (the barrier height does not depend on the substrate carrier concentration) for a silicon substrate resistivity higher than 0.5 Ω-cm (or a carrier concentration lower than 8×1015 cm-3) 4. 4 ITO/n-Si solar cells: design, fabrication and characterization The solar cells were fabricated using... temperature, as shown in the insert of Figure 15 -1 10 1x10 -11 ± 2 J02 [A/cm ] ± 1x10 10 -17 10 -2 10 E0d = 1.21± 0.02 eV - 14 -20 ± 3.5 4. 0 4. 5 5.0 -1 n = 1.02 (J02) -3 10 n = 2.26 (J01) -4 1x10 -5 1x10 0,96 Barrier Height [eV] 2 JSC [A/cm ] 1000/T [K ] 0, 94 0,92 0,90 160 200 240 280 Temperature [K] -6 10 0 5 10 15 qVOC/kT 20 25 Fig 15 Measured dependence of Jsc on Voc at room temperature, and calculated... Mater Lett., Vol.61, N. 14- 15, 3030-3036, ISSN 0167-577X Nagatomo, T.; Endo, M & Omoto, O (1979) Fabrication and characterization of SnO2/n-Si solar cells, Jpn J Appl Phys., Vol.18, 1103-1109, ISSN 0021 -49 22 Nagatomo, T.; Inagaki, Y.; Amano, Y & Omoto, O (1982) A comparison of spray deposited ITO/n-Si and SnO2/n-Si solar cells, Jpn J Appl Phys., Vol.21, N 21-2, 121-1 24, ISSN 0021 -49 22 Nakasa, A.; Adachi,... John Wiley and Sons, ISBN 978 047 1 143 239, N.Y Tarr, N & Pulfrey, D (1979) New experimental evidence for minority-carrier MIS diodes Appl Phys Lett., V. 34, N .4, 15 February 1979, 295-297, ISSN 0003-6951 Theuwissen, A & Declerck, G (19 84) Optical and electrical properties of reactively d c magnetron-sputtered In2O3:Sn films Thin Solid Films, Vol.121, N.2, 109-119, ISSN 0 040 -6090 Trmop, R.; Hamers, R & . shown Solar Energy 88 200 40 0 600 800 1000 0 20 40 60 80 100 b c a Transmittance [%] Wavelength [nm] a- F/Sn-0 b- F/Sn-0.50 c- F/Sn-0.85 0.00.20 .40 .60.81.0 4. 3 4. 4 4. 5 4. 6 4. 7 F/Sn. spray 26.0 90 13 .4 Gouskov, 1983. spray 9. 34 85 21.0 Vasu et al., 1990 spray 10.0 90 34. 9 Manifacier, 1981 spray 4. 4 85 44 .7 Saxena, 19 84 sputtering 12.5 95 47 .9 Theuwissen, 19 84 evaporation. polycrystalline configuration with a (40 0) preferential grain orientation. 10 20 30 40 50 60 70 0 2000 40 00 6000 8000 (622) (611) (44 0) (41 1) (40 0) (222) T =48 0 °C [Sn]/[In]=0 % [Sn]/[In]=5